by Bill Burkhardt, EPEHA Member
(February 1998)
You don't have to be an engineer to see interesting things in the Friendship Park sewer project. How does boring a tunnel through rock sounds for starters? Needless to say, sewer laying methods have come a long way in the last few decades.
Most people in El Prado know that our tract's main sewer, which leaves the tract at the lower end of Tarapaca, broke three years ago due to land movement. The new sewer line is being routed across Friendship Park to avoid the unstable land problem that destroyed the original sewer.
The City of Rancho Palos Verdes is contracting with Los Angeles County to design and build the sewer. Excavation for the sewer pipe will be a combination of trenching and boring.
Yes, boring: a three foot diameter, mechanical "earthworm" will bore about 1000 feet of the new sewer line. Most of the 1000 feet will be through our Monterey shale (a.k.a. Altimira shale), a crumbling, brown sedimentary rock that we have often seen during our El Prado landscaping efforts.
The new sewer will be graded for gravity flow over its entire length. The high end of the new sewer will connect to the existing sewer at the south end of Tarapaca Road. It will run parallel to Tarapaca for nearly 500 feet and then go northeast to connect to the sewer line at Rue Le Charlene. The boring subcontractor has already finished the short section between Rue Le Charlene and a newly constructed manhole just inside the Park's north border fence. He is currently boring the 480 foot section under the upper meadow.
You can see what's going on currrently near the upper park restroom. Take a short walk down the park maintenance road, starting at the lower end of Calle Aventura. When the boring subcontractor finishes this current section in a few days, he will start on the second long tunnel section (460 feet) parallel to Tarapaca.
Anatomy of the "earthworm"
On your walk into the Park you will probably see several large steel cylinders that comprise the "skin" of the mechanical "earthworm". These steel cylinders are currently stacked above the site of the current boring activity. They are three feet in diameter and 20 feet long. They are black with a patina of rust forming. Don't confuse them with the much smaller (10 inch ID) clay colored, ceramic sewer tiles lying nearby.
The first long tunnel, which the subcontractor is currrently boring, begins at the Park's upper restroom. Since the sewer line will be deep in the earth (more than 30 feet deep in some places) the boring must begin in a deep pit, termed a "jacking" pit because of the hydraulic jacks that force the earlier described steel cylinders into the new tunnel. You can recognize the pit by the temporary chain link fence which surrounds it and keeps errant dogs, children and park visitors out of harm's way.
Those of you at the homeowners' annual meeting last November heard the speaker describe the sewer line tunnel as 24 inches in diameter rather than 36 inches. The diameter had to be increased to 36 inches because the ground had much more rock than was originally anticipated and the boring technique required a larger diameter to excavate the larger chunks of rock. The equipment necessary to power the current 36 inch diameter boring operation is reportedly the largest available for sewer line boring. It was custom built for the subcontractor.
After the jacking pit by the upper restroom was dug, the subcontractor lowered the equipment to power the boring operation in place on the floor of the pit. He then lowered the first 3 foot diameter steel cylinder. A large steel auger, the leading edge of which has digging "teeth", had been fit inside this head-end cylinder.
How the "earthworm" eats
As a large electric motor turns the auger inside the cylinder, hydraulic jacks press the auger and the steel sleeve into the dirt/rock face along the designed sewer route. The auger carries the excavated dirt/rocks along its length inside the steel casing and discharges it into the jacking pit. The discharged dirt/rock is then removed as it accumulates by a large scoop on the crane you will see towering above the top of the jacking pit.
How the "earthworm" grows
After the first section of the "earthworm" dug its way entirely into the dirt/rock, the contractor lowered another 3 foot diameter steel cylinder, also containing another section of auger inside it, into the pit and positioned it behind the cylinder that had recently dug its way into the hole. He attached the new auger section to the preceding auger section, welded the front end of the new steel cylinder onto the back end of the preceding steel cylinder and re-commenced the digging.
As each 20 foot section of the "earthworm" digs its way into the dirt/rock under the meadow, a new section is added until the tunnel reaches a "receiving pit" on the far side of the meadow, 580 feet from the starting point!
Will the tunnel come out in the right place?
Sheesh ... you want to know everything, don't you! Well, that's a good question. The mechanical "earthworm" does not meander the way its living namesake probably would. The operator has some control over its path. Periodically during the drilling, the operator will withdraw the auger string and check the progress. He has a couple tools to gauge the accuracy of the tunnel path. One is a laser that beams a light from one end of the tunnel to the other. The other is a liquid filled tube that goes through the tunnel and has sensors at either end. (Please pardon the vagueness here; your reporter doesn't know just how these two tools are used to gauge the path of the tunnel.) The operator can adjust the boring path by adjusting four vanes located on the leading end of the "earthworm". Then it will accurately break through at the targeted receiving pit.
Putting in the ceramic sewer pipe
Once the tunnel between the jacking pit and the receiving pit is complete, we must abandon our "earthworm" simile. The auger, which fills the steel tunnel lining, is removed, leaving the 3 foot diameter steel liner in place permanently.
The ceramic sewer tiles, bound by stainless steel bands to 4 inch by 4 inch timbers, are pulled into the tunnel from the receiving pit individually. Each tile will be set with the "bell" end upstream. The line of sewer tiles will be aligned manually by a workman in the tunnel so there will be no low spots or high spots.
Concrete manholes will be built in each pit and the new sewer line will be connected to them at each end of the tunnel. Another concrete manhole will be built in the middle of the meadow for access to the sewer. There will be a total of seven new manholes built for the new sewer line.
Filling the tunnel
When everything is in place, the empty space between the steel tunnel lining and the new sewer pipe will be filled with sand. The sand will be pumped in through a PVC (plastic) pipe that runs the length of the tunnel and is gradually withdrawn as the sand fills the empty space.
Sewer line sections between the tunnels will be laid in trenches. Going across gullies, the trenches will not need to be as deep as the tunnels over most of their length.
Archaeological, environmental sites to be protected Thanks to the boring technique, a minimum disturbance of surface archaeological and environmentally sensitive sites will occur. Orange storm fencing and protective boundary tape mark the sensitive sites, and an archaeologist is responsible for monitoring the operation from beginning to end. So take a short hike into the Park from time to time and monitor the progress yourself. And don't put it off until summer. It might just be finished by then, and you'll miss all the fun!